This invention relates generally to laser diode arrays and, more particularly, to high power laser diode arrays used for pumping fiber lasers. Optical fiber lasers are often employed as components in high power laser systems and, in a typical configuration of a fiber laser, an array of laser diodes is used for optically pumping the fiber laser. Such arrays are often referred to as laser diode bars.
A laser diode bar structure has a significant disadvantage in that light emitted from the structure consists of multiple relatively wide emitting stripes arrayed along a straight line. For example, each emitting segment may be approximately 1 μm×3.5 μm in cross section. A typical array may have an emitting surface with about nineteen or twenty segments, with an overall width of about 1.0 cm and with each segment only 1 μm in height. Therefore, the output beam from the laser diode array is highly asymmetrical. The raw, unfocused output beam is highly divergent and, even after focusing, the beam suffers from two related asymmetries: an elongated elliptical beam cross section, wider in a lateral direction across the width of the array than in a perpendicular transverse direction, and astigmatism, manifested in different divergence profiles for the lateral and transverse directions. These asymmetrical properties are highly undesirable for most applications, including supplying pump power to a fiber laser.
Various solutions have been proposed to address the asymmetry of output from a laser diode array, such as using twisted cylindrical lenses or multiply faceted mirrors, but all have potential disadvantages, such as having inherent energy losses or being difficult to manufacture.
One proposal in a paper by Peter Y. Wang, entitled “Beam-shaping optics delivers high-power beams,” Laser Focus Magazine, December 2001, suggests the use of two sets of roof prisms, which together effect reshaping of the array output beam by using the first set of prisms to rotate each elemental diode output sub-beam by 90° and then using the second set of prisms to stack the sub-beams in a more symmetrical configuration as a composite output beam.
An international patent publication, identified by publication number WO 2004/023186 A1, entitled “An Apparatus for Converting Light Beams,” filed in the name of inventor Yixuan Xiao, discloses using a plurality of parallel angled mirrors to shape sub-beams output from a linear array of light emitters.
Unfortunately, neither of these proposed solutions is completely satisfactory. The need for two sets of roof prisms as proposed in the Wang paper results in higher cost and potential alignment difficulties. The use of mirrors as proposed in the Xiao patent application potentially results in higher energy losses and manufacturing costs when compared with the use of prisms. Moreover, neither solution satisfies another important concern in the use of diode laser arrays in high-power applications. Each diode laser array must necessarily be cooled and, of course, supplied with electrical power. The need for cooling and electrical connections to the diode laser array means that the output light beam must be directed such that it does not interfere with the cooling and electrical connections.
Accordingly, there is still a need for an optical structure that reduces the asymmetry of an output beam from a laser diode array and avoids the drawbacks of the prior art. The present invention is directed to this end.